1. Field of the Invention:
The present invention relates to novel polyamide-based compositions having improved properties of impact strength, ductility and crystallinity, together with markedly enhanced heat stability. The present invention also relates to processes for preparing such compositions.
2. Description of the Prior Art:
Unmodified polyamides which possess high break strengths are considered to be tough polymers. In contrast, same are typically rather lacking or deficient as regards cracking and resistance to the propagation of cracks, which is the result of a certain sensitivity to chipping or notching and, possibly, of breakage due to embrittlement. This tendency to break as a result of being brittle rather than ductile substantially limits the uses of such polyamides and their reliability.
Improvement in the impact strength of thermoplastic polyamides has been studed in detail and numerous solutions to the problem have been proposed. Thus, British Pat. No. 998,439 features a composition comprising a mixture of 50 to 99% of a linear polyamide and 1 to 50% of an olefinic copolymer containing from 0.1 to 10 mol percent of acid functions, dispersed in the polyamide in the form of particles having a diameter of less than 5 microns.
U.S. Pat. Nos. 3,388,186 and 3,465,059 feature mixtures of polycaproamide and olefin copolymers containing from 1 to 20 mol % of (meth)acrylic acids or derivatives thereof, onto which an aminoacid has been grafted.
U.S. Pat. No. 3,668,274 describes mixtures of polyamides and 2.5 to 30% of a multiphase polymer comprising carboxyl functions, consisting of a first elastomeric phase (50 to 99.9% of an alkyl acrylate and butadiene) onto which a more rigid copolymer, containing from 1 to 50% by weight of an unsaturated carboxylic acid, has been grafted.
U.S. Pat. No. 3,845,163 describes mixtures of 60 to 80% by weight of a polyamide and 15 to 40% by weight of an .alpha.-olefin copolymer containing from 1 to 8 mol % of an .alpha.,.beta.-ethylenic carboxylic acid in which at least 10% of the acid functions have been neutralized by metal ions.
French Pat. No. 2,311,814 features a very large number of multiple-phase polyamide compositions having improved toughness, which consist of mixtures produced in the molten state from 60 to 99% by weight of a polyamide and 1 to 40% by weight of at least one other phase containing particles of at least one polymer having a modulus under traction of less than 1/10 of that of the polyamide, which "adheres" to the polyamide and the particle size of which is between 0.01 and 3 microns. The polymers which can be used as the disperse phase are chosen from among the non-crosslinked, linear or branched thermoplastic or elastomeric polymers belonging to the following chemical families:
(i) Copolymers of unsaturated monomers and molecules which generate adherent sites, such as: carbon monoxide, carboxylic acids with .alpha.,.beta.-ethylenic unsaturation, or derivatives thereof, unsaturated epoxides and residues of an aromatic sulfonylazide substituted by carboxylic acids;
(ii) Polyurethanes derived from polyester-glycols or polyether-glycols; and
(iii) Polyether network polymers obtained by reacting epoxide monomers.
The majority of the polymeric additives mentioned or noted in the prior art have a relatively low thermal or chemical stability, and this limits their use in the field of conventional polyamides, in which the conditions of preparation or processing are very harsh. Thus, the polyurethanes and polyether network polymers described in French Pat. No. 2,311,814, supra, can reasonably be used only in conjunction with matrices having melting points below 200.degree.-220.degree. C.
The very large number of copolymers of olefins and acrylic derivatives described in the prior art also undergo very substantial decomposition when they are mixed in the molten state with polyamides having a high melting point, such as, for example, polyhexamethylene adipamide.
The use of polyamides which exhibit good heat stability as a reinforcing phase for thermoplastic compositions comprising a polyamide matrix, has not been heretofore described. Mixtures of conventional polyamides or copolyamides, among which may be mentioned without limitation, polycaprolactam, polyhexamethylene adipamide, polyhexamethylene sebacamide, polyhexamethylene dodecanamide and their copolymers, in fact have a well-known chemical instability in the molten state which is due to the existence of exchange reactions between the different types of amide sequences. It is thus not generally possible to obtain, in the molten state, a stable dispersion of particles of a polyamide or copolyamide in another polyamide since such a system inevitably changes rapidly into a homogeneous copolyamide initially formed of polyamide blocks, which themselves change with moderate rapidity into a statistical copolyamide.
Polyamide additives which have been used to modify other polyamides, generally for purposes other than that of creating stable multiphase compositions, are found in the prior art. U.S. Pat. No. 3,645,932 claims the nucleation of polyamides derived from polymeric fatty acids by means of 0.1 to 10% by weight of a nucleating agent, which can be another polyamide having a high melting point and possessing a high tendency to crystallize.
U.S. Pat. No. 4,062,819 features mixtures of thermoplastic polyamides possessing improved rheological and flow properties which comprise from 80% to 99.99% by weight of a polyamide resin and from 0.01% to 20% by weight of a polyamide additive which has an amine number of less than 3 and is prepared by reacting an aliphatic or cycloaliphatic diacid having from 18 to 52 carbon atoms and a saturated aliphatic diacid having from 2 to 13 carbon atoms, which can represent up to 30% by weight of the acids, with a stoichiometric amount of one or more saturated aliphatic diamines having from 2 to 10 carbon atoms. The aforedescribed polyamide additives have relatively high melting points and are essentially compatible with the base polyamide resins. The resulting polyamide mixtures, besides improved flow properties possess a slightly greater flexibility and a crystallization rate which is not significantly lower than that of the matrix when the proportion of additive does not exceed 10% by weight. The impact strengths of these compositions, however, are not significantly greater than the impact strengths of the base polyamides. Indeed, in the case of polyamide-6,6, the impact strength is reduced by 14% when the composition comprises 2% by weight polyamide additives. Moreover, in the case of polyamide-6, the impact strength increases by 9.5% with 2% by weight additives, but drops to 15.5% below that of the base resin with 10% by weight additives.